A so-called “lost-foam” casting process is a well-known technique for producing metal castings. A fugitive, pyrolizable, polymeric, foam pattern (including casting, gating, runners, and sprue) is covered with a thin (typically in the range of 0.25–0.5 mm), gas-permeable refractory coating/skin such as mica, silica, alumina, or alumina-silicate, for example. The pattern is embedded in compacted, unbonded sand to form a mold cavity within the sand. Molten metal is then introduced into the mold cavity to melt, pyrolyze, and displace the pattern with molten metal.
Gaseous and liquid decomposition/pyrolysis products escape through the gas-permeable, refractory skin and into the interstices between the unbonded sand particles. Typical fugitive polymeric foam patterns comprise expanded polystyrene foam (EPS) for aluminum castings and copolymers of polymethylmethacrylate (PMMA) and EPS for iron and steel castings, for example.
The polymeric foam pattern is made by injecting pre-expanded polymer beads into a pattern mold to impart the desired shape to the pattern. For example, raw EPS beads (typically 0.2 to 0.5 mm in diameter) containing a blowing/expanding agent (e.g. n-pentane) are: (1) first, pre-expanded at a temperature above the softening temperature of polystyrene and the boiling point of the blowing agent; and (2) molded into the desired configuration in a steam-heated pattern mold which further expands the beads to fill the pattern mold. Complex patterns and pattern assemblies can be made by molding several individual mold segments, and then joining the mold segments by gluing, for example, to form the pattern or pattern assembly.
The filling of a “lost-foam” casting with molten metal is typically achieved with a gravity-cast or a countergravity-cast method. In a gravity-cast lost-foam process, an overhead ladle or furnace pours metal into a pouring basin and sprue which is in communication with the casting pattern. The metallostatic head in the basin and sprue is the force driving the metal into the casting pattern. In a countergravity-cast lost-foam process, an applied pressure drives the molten metal into the pattern. This pressure can be applied in the furnace vessel, which sits below the pattern, or in the pattern flask itself.
There are three categories of gating systems in gravity-cast lost-foam process which are based on the orientation of the metal front as it enters the casting pattern. These categories are top-fill, bottom-fill and side-fill gating. A top-fill gating system has the sprue and runners located above the casting pattern. This causes molten metal to flow downward against the casting foam pattern. A bottom-fill gating system has runners which are located below the casting pattern. The metal flows downward though the vertical sprue, but flows upward against the foam casting pattern. A side-fill gating has a plurality of runners along the length of a sprue and casting pattern. The vertical sprue may be flanked by two or more patterns for making multiple castings with a single pour. Typically, a side gated foam pattern has a complex metal front of varying orientations.
Bottom-fill casting is often preferred in lost foam castings. The advantage of bottom gating is a reduction in gas bubbles that make their way into the casting causing voids or porosity defects. Coatings employed during lost foam casting often cannot absorb all the foam decomposition products as quickly as they are produced. If the molten metal is above the foam, as in top-fill casting, the gas bubbles move upward through the molten metal and collect at a top surface thereof. These gas bubbles lead to subsurface void defects in the casting. If the molten metal is below the foam, as in bottom-fill casting, the gas merely collects and slows the molten metal front movement. Thus, defects are minimized when using the bottom-fill configuration.
However, disadvantages do exist when using bottom-fill, gravity cast systems. The vertical sprue required in bottom-fill casting is typically formed using a foam pattern having the shape and configuration of the desired final shape and configuration of the sprue. Thus, the molten metal must still travel through a long section of thick foam to reach the casting area. Undesirable gases are created, but are unlikely to be carried into the casting area. However, oxide films are created which travel with the molten metal into the casting resulting in a reduction of a fatigue life of the metal. Thus, it is desirable to reduce or eliminate the foam used in the sprue to optimize the material properties of the casting.
Commonly owned U.S. Pat. No. 6,619,373 B1 is incorporated herein by reference to provide additional background and provide an example of other attempts at providing a solution for the above-mentioned disadvantages.
It would be desirable to develop a method and apparatus for forming a sprue for receiving molten metal and directing the molten metal to a foam pattern in a mold cavity for a lost foam casting process used in producing metal castings, wherein the sprue facilitates a minimization of production costs and an optimization of material properties of the resultant casting.